1. Technical Field
The present invention generally relates to the technical field of semiconductor package assembly.
More in particular, the present invention relates to a method of assembly of a semiconductor package comprising the step of treating the electrical contacts thereof for the improvement of the electrical test yield on the packages so obtained, with particular reference to ball grid array (BGA) packages.
The following description is made with reference to the specific technical field of BGA semiconductor packages for the sole purpose of simplifying the disclosure of the invention. However, the invention is applicable to all types of semiconductor packages, for example lead frame-based, pin-based, flat contact packages, etc.
2. Description of the Related Art
In the past few years, ball grid array (BGA) semiconductor packages have enjoyed widespread use and success in the industry thanks to the many advantages they offer compared to the more traditional lead frame packages. The most obvious advantage they confer is their ability to host an increased number of interconnections within small dimensions whilst maintaining satisfactory ease of use and safety levels. This feature enables them to be used successfully in many high performance applications such as microprocessors, controllers, memories and chip sets, which have high density interconnection packages.
The basic architecture of a typical BGA package comprises a solder ball base, providing external electrical interconnection with the rest of the system, underlying a substrate, usually made of an inorganic material, e.g. silicon, resin or glass, which in turn underlies a die, as shown for instance in FIGS. 1 and 2, where the BGA package is globally indicated with 1.
The solder balls 2, which functionally replace the leads used in lead frame or pin grid array (PGA) packages, are attached to metal pads 4 at the bottom of the substrate 3 and their composition may include, for example, copper, tin, silver, lead, or bismuth. Typically, a 10×10 mm BGA package will contain up to 300 solder balls.
Solder balls are mechanically tougher than leads, thus enabling the package to better tolerate rough handling. Also, ball arrays allow for slightly imperfect placement during assembly, as they are, to a certain extent, capable of self-alignment to their attachment sites.
In the assembly of BGA packages, solder balls are typically placed on metal pads 4 (integrated in the substrate 3) atop a layer of flux liquid and passed for reflow. The solder ball 2 attaches to the metal pad as it melts and re-solidifies, as shown for instance in FIG. 3.
In this process, which is standard, solder paste, a sticky mixture of flux and solder, is first applied to all the metal pads 4 with a gold-plated stainless steel tooling.
The substrates 3 then enter a pre-heat zone, where the temperature of the substrates and all their components is gradually, uniformly raised. The substrates then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the solder to the metal pads 4 on the substrates. The surface tension of the molten solder helps keep the solder in place. There are a number of techniques that can be used for reflowing solder: infrared lamps, hot gas, fluorocarbon, nitrogen, to name a few.
After soldering, the substrates 3 are washed to remove flux residue and any stray solder balls that could short out closely spaced metal pads. Water soluble fluxes are removed with deionized (DI) water and detergent, followed by a rinsing step with DI water, and an air blast to quickly remove residual water.
Finally, the substrates are visually inspected for missing or misaligned solder balls and solder bridging. If needed, they are sent to a rework station where a human operator corrects any errors. The substrates, inclusive of the reworked substrates, are then sent to the testing stations to verify that they work correctly.
The cleaning step is important, as poor cleaning can result in non-conductive residues on the ball, which in turn would hinder the electrical conductivity of the device and hence give poor yield during electrical function tests on the device.
Solder balls, particularly those in tin-rich alloys, are susceptible to oxidation or contamination after cleaning. This occurs naturally upon contact with atmospheric oxygen and water vapor and also upon handling and stacking, which expose the solder balls to accumulation of foreign material.
This degrades the solder finish and hence leads to a loss in the electrical testing yield, particularly in high temperature and/or high humidity treatments such as mold compound curing or storing at atmospheric conditions.
By “electrical testing yield” is meant the proportion of devices from a production lot, following integrated circuit (IC) assembly, found to work properly upon electrical testing.
Electrical testing generally consists of a method of IC testing through an electronic tester whereby a probe is pressed against the solder balls of the IC. Bad chips are segregated for re-test before scrapping. The outcome of electrical testing depends on the quality of the solder ball to test pin conduction. Numerous electrical tests are dependent on precise resistance, current flows and time delays. Hence if an IC is fully functional but cannot communicate perfectly with the testing socket it will result in a false failure. These devices may be recovered by re-testing them after cleaning the socket or device contacts.
Various cleaning chemicals are sometimes employed to improve the cleaning of the solder balls but, even when effective, the clean solder surface then remains unprotected and its quality degrades over time and with use.
Different solder alloys, based on a tin-rich metal composition, can also be used for different applications, based on different requirements. The composition of the solder alloy, in fact, affects the ball shear force, that is, the strength of the adhesion of the balls to their respective metal pads.
Another characteristic being affected by the composition of the solder alloy is the melting point of the solder. A standard tin/lead (63%/37% (w/w)) composition, for example, melts at about 183° C. while a pure tin (Pb-free) composition melts at 232° C. This requires higher temperatures and longer time to solder, and subjects the materials to higher stress, due to the higher temperatures. A typical lead-free solder incorporates silver, copper, and other metals in the range of 1-4%, which causes it to melt at around 218° C. This is advantageous in terms of energy and time, and, additionally, improves joint strength and reliability.
Other parameters affected by the solder alloy composition are the compatibility with PCB solder pastes, and the resistance shown in the drop-test.
The drop-test is a test mostly used in portable electronics, such as mobile phones, where a complete assembled set is dropped several times in a ‘bumping’ machine to mimic the phone being dropped. These mechanical shocks test the joint strength between the IC and the board, and the ability of the device to maintain a reliable electrical path.
However, some alloys, which incorporate elements to minimize surface insulation, tend to be expensive and less consistent in performance due to the very low concentrations of the metals incorporated (e.g. germanium), typically at less than 100 ppm. An example is SnAg with 50 ppm of Ge.
Yield loss at the electrical testing stage is not cost-efficient, as the final product at this stage of assembly has high added value and materials. Poor yield will require the failed devices to be re-tested perhaps several times, to recover the good devices that only failed due to poor contact between solder ball and test pin. This results in poor efficiency of the test equipment utilization, mainly due to longer cycle times (that is, the time taken to test the batch), a higher risk of mixing up good and faulty items, and excessive handling.
Ideally, in fact, good or bad devices are immediately identified from a first and only test. Re-testing to recover good devices which were falsely found bad due to poor chip to socket contact, represents a waste of time. Also, in an IC production facility, several different Si chips may be packaged in physically identical ICs. As re-testing generates an increasing number of small sub-lots, the probability of different devices getting mixed up increases with the number of re-testing cycles required.
This directly implies that an improved first pass yield (first test attempt) would result in considerable financial and qualitative gains.
To overcome the problem of solder ball contamination, chemical methods of passivation or pore blocking of the metal surface have been tried, such as the ones using chromic and phosphoric acids. However, these chemicals are ineffective, since the protective oxide layer generated is relatively thick and has poor electrical conductivity.
U.S. Pat. No. 6,863,718 (Lamborn et al.) discloses the use, as a coating for metal pigment particles, of a product resulting from the reaction between an organic phosphonic acid with an amine having at least one organic group containing at least six carbon atoms, to inhibit their reactivity to water.
Shogrin et al. (NASA/TM, 2001-210947, 1-11) describe the passivation of stainless steel surfaces, more in particular lubricated contacts, with one of four techniques: high and low temperature chromic acid bath, a tricresyl phosphate (TCP) soak, or UV/Ozone treatment for 15 minutes. The lubricant of the contacts is perfluoropolyethers (PFPEs). It is concluded that the PFPE lubricated lifetime (Krytox 16256) of sliding 440C stainless steel couples does not statistically change as a result of the various passivation techniques. Also, PFPE lubricated lifetime (Brayco 815Z) of 440C stainless steel couples in boundary lubricated rolling contact do not statistically change as a result of the TCP passivation technique.
U.S. Pat. No. 5,304,257 describes a process for preparing a corrosion-resistant trivalent chromium coating on aluminum and aluminum-alloy substrates which comprises treating the substrates with an acidic aqueous solution free of hexavalent chromium and contains from about 0.2 to 3.0 grams per liter of a water soluble trivalent chromium compound, from about 0.05 to 1.5 grams per liter of a water soluble fluoride compound and a sufficient amount of an alkaline reagent to maintain the aqueous solution at a pH ranging from about 4.0 to 5.5 to form the trivalent-chromium coating on said aluminum substrates.
The above examples give protection but poor conductivity, a condition which is, of course, undesirable in electrical testing. Moreover, the traditional method of only washing with hot DI achieves the cleaning of the surface but not the protection of the surface from the eventual contamination and oxidation.
The need thus arises of providing a cheap, simple and reliable method of assembly of a semiconductor package comprising the step of treating the electrical contacts thereof, in particular the solder balls of a ball grid array (BGA) semiconductor package, to obtain an increase in electrical testing yield of the semiconductor packages thus assembled by preventing false electrical failures due to poor contacting between device and test socket.